In step with industrial progress, water pollution and water resources scarcity are emerging as severe problems. Because of industrial advancement, population growth, and increased standards of living, the demand to good quality water is increasing rapidly. However, water pollution due to domestic waste or industrial sewage has become a serious problem and therefore, available water has become scarce. In order to utilize limited water resources efficiently, purification treatment is absolutely necessary before drinking natural water is to be consumed, in addition to the removal of sources of water pollution.
Conventional water purifiers take forms of different kinds of purification systems depending on filter type. At present, the purification system using a filtration membrane is considered to be the most effective because it can eliminate impurities including minute substances such as bacteria and heavy metals.
As representative filtration membranes for use in water purifiers, there exist an ultrafiltration membrane, a nanofiltration membrane and a reverse osmosis membrane. Among them, the ultrafiltration membrane is used to remove mainly colloid-sized substances. Although it can provide a high flow rate due to larger pore size than those of the nanofiltration and reverse osmosis membranes, there is a limit to elimination of minute substances such as bacteria and heavy metals.
Meanwhile, the reverse osmosis membrane can eliminate almost all minute substances such as bacteria and heavy metals and thus has been widely used recently. Such reverse osmosis membrane based water purification systems are disclosed in several documents including U.S. Pat. Nos. 4,626,346 and 6,190,558. However, most such reverse osmosis membrane based water purifiers are provided with storage tanks for containment of purified water, due to insufficient flow rate. Therefore, there are problems in that the volumes of the purifiers are increased due to installation of the storage tanks, and microorganisms are propagated due to prolonged storage, thereby causing secondary pollution.